Composition for cleaning and polishing aluminum and aluminum alloys



awn:

3,041,285 COMPOSITION FOR CLEANING AND POLISHING ALUMINUM AND ALUMINUM ALLOYS Kenneth Walter Newman, Van Nuys, Calif., assignor,

by mesne assignments, to Purex Corporation, Ltd., a

corporation of California No Drawing. Filed May 27, 1955, Ser. No. 511,779

7 Claims. (Cl. 252-149) This invention relates to chemical compositions for the cleaning and polishing of aluminum and aluminum alloys.

Principally, the object of the invention is to provide a chemical cleaner which will remove oxide, tamish nd other discolor-lug materials or soil fromthe surfaces of aluminum and aluminum alloys, without attacking the underlying pure aluminum or aluminum alloy surfaces.

It is therefore a further and very important object of the invention to provide a chemical brightening or polishing agent for aluminum and aluminum alloys which will not attack the metal and therefore will not weaken the metal surfaces or promote danger of structural failure.

A still further object is to provide a polisher or cleaner of the indicated nature which is especially adapted to the polishing of aircraft surfaces which are formed from aluminum and aluminum alloys. Such a composition must be in all respects safe, not in any way harmful to personnel who apply it, and not in any way corrosive to the base aluminum or aluminum alloy metal, even after water or other solvent employed is evaporated and even after the lapse of a considerable period of time, such as several weeks, wherein corrosive action, as between faying or contacting surfaces, might develop.

Other objects of the invention, as well as the various features of novelty therein, will become apparent to those skilled in the art upon reference to the following specification wherein certain preferred embodiments are disclosed.

Compositions of this invention include essentially thiourea and 'a relatively large proportion of a particular non-corrosive u I l 1 of yleling a pH below 3. The particular acid is glycolic aci proportion of this acid is at least eqTfal'To that of the thiourea (by weight) and preferably considerably more, such as one and one-half times the thiourea, or twice, or even up to three or four or five times the thiourea. In aqueous solutions employing around 10 to 20 times as much water as the acid, a pH between 2.0 and 2.8 is yielded without being corrosive to aluminum or aluminum alloy surfaces. Such proportions of acid and thiourea are readily soluble in water, the solution is generally free from objectionable odor, and the solution also is relatively non-toxic to personnel employing the materials. Since thiourea is soluble in water to the extent of only 9% to 10% by weight, and since a considerable concentration of the acid is required, the high water solubility of this acid, at least to the extent of being soluble in an equal weight of water, is important. This solubility is additionally important when it is considered that a thick t is employed in the solution in order that the polish may be sprayed upon the metal or mopped upon the metal and be allowed to remain, without running off, for a considerable period of time, such as 15 minutes to 30 minutes or longer, before the cleaner is washed off or otherwise appropriately removed from the surface being cleaned.

While metal cleaning solutions containing thiourea and and acids have been known heretofore, none have been satisfactory for thorough cleaning of aluminum and aluminum alloy surfaces, such as aircraft surfaces, particularly because of being highly corrosive to such surfaces.

t- Umted States Patent ice 3,041,285 Patented June 26, 1962 In practice one part of thiourea is commonly employed with one and one-half to two or three parts by weight of glycolic acid, and these are used in about ten parts by weight of water to yield the required solution. Ordinarily, the solution has added thereto a thickener in quantity to produce the desired consistency above described. Such a thickener might comprise about one-fourth part of methyl cellulose, or ethyl cellulose, or gum tragacanth, or hydroxyethyl carboxymethyl cellulose, or agar agar, or other similar thickener which will dissolve in the solution and will subsequently wash off readily. As one useful example, about two parts by weight of glycolic acid is used with one part by weight of thiourea and these are dissolved in about ten parts by weight of water (sufiicient to dissolve the thiourea), such solution yielding a pH around 2.2 to 2.5. One-fourth part by weight of methyl cellulose is added to yield a thickened solution which may be applied as such by spraying, brushing, or otherwise as desired.

In a particular preferred instance I have employed the following composition (by weight):

Percent Glycolic acid 11 Thiourea 6 Water 81.7 Methyl cellulose 1.3

Another example of a desirable composition is the following (by weight):

Percent Glycolic acid 7.5 Thiourea 5.0 Water 86.0 Methyl cellulose 1.5

Approximate Parts and Percentages TABLE 1 Parts Per- Parts Per- Parts Pcr- Parts Percent cent cent cent 1 8 1 8 2 15 3 20 1 7 1 8 l 7 1 7 1O 85 1O 82 10 75 10 71.5 Thickener M 2 $6 3 $4 1.5

TABLE II Parts Per- Parts Per- Parts Per- Parts Percent cent cent cent Acid 1 6 2 11 4 20 4 25 Thiourea l 6 1 5.5 1 5 1 6 Water 15 85 15 S3. 5 15 75 10 Thickener l 3 3g 4 TABLE III Parts Per- Parts Per- Parts Per- Parts Percent cent cent cent Acid 1 5 2 10 3 13 4.3 16 Thi0urea 1 5 1 5 1 5 1 4 Water 20 90 20 83 20 20. 77 Thiolrenor K 2 3 2 .7 3

While the above tables, showing approximate parts and percentages, indicate as little as 5% of glycolic acid with not more than an equal amount of thiourea, and a percentage of glycolic acid as high as 25% of the composition (4 to 5 times the thiourea), it is generally preferable to employ appreciably, but not greatly, more glycolic acid than thiourea, such as approximately twice as much as in the preferred example first listed above, or around one and one-half times as much as in the second example. As to the thickener, while about 1.5% is usual, more or less, or none at all, as indicated, is acceptable under varying conditions.

The possible over-all percentage ranges for the constituents, as represented by the examples and tables above, are thus as follows (by weight):

Glycolic acid, about 5% to 25% Thiourea, about 4% to 8% Water, about 65% to 90% Thickener, to 4% While a limit of about 25 of glycolic acid has been indicated, it nevertheless would be possible sometimes to employ up to about 40% glycolic acid. In such a case, the water content would be about 55% to dissolve satisfactorily of thiourea, this composition thus employing about nine parts of glycolic acid, one part of thiourea, and twelve parts of water.

The indicated acid proportions ordinarily yield pH values between 2.2 and 2.5, pH 2.0 to 2.8, that is less than about pH 3, being necessary to effect adequate cleaning to yield the required brightened metal surface.

From the standpoint of appropriate materials to clean aluminum and aluminum alloys, it is to be recognized that aluminum is an amphoteric metal and that its action is quite different from that of other metals. Thus, it is said in the work of Jenny and Lewis, entitled Anodic Oxidation of Aluminum and Its Alloys, published by Chemical Publishing Co., Inc, New York, N.Y., in 1940, pages 50 and 51, that:

In its behavior toward acids, aluminum differs greatly from other metals. It is not the degree of dissociation of an acid, but the aflinity of the aluminum for the anion which is the determining factor in the solution process.

As a consequence it will be appreciated that the acid above disclosed as providing for the proper cleaning of aluminum and aluminum alloys, without corrosion of the base metals, is an acid whose anion does not have corrosive affinity for aluminum and its alloys, which is nonoxidizing, and yet which, in the required concentrations, provides the indicated 2.0 pH to 2.8 pH to dissolve the various oxides, sulphides and other staining and soiling constituents which must be removed to yield the bright or polished surface which is required and which, as produced by the present invention, glistens and gleams as though the surface had been hand polishd and hand buffed. Such a condition is accomplished merely by mopping or spray ing onto the metal surface a composition as above described, which necessarily contains glycolic acid as the acid constituent, allowing it to remain on the metal surface for a period of from about fifteen minutes to about thirty minutes, scrubbing the surface preferably with a stiff brush or similar rubbing instrument, and then rinsing the surface thoroughly with water, as from a hose, whereby to leave the surface clean and polished. When the composition is to be mopped or wiped on the surface to be cleaned, it preferably contains an indicated thickener, though not necessarily so. When the solution is to be sprayed on the surface, the thickener might be omitted, though not necessarily so because the thickener restricts run-off and also evaporation of the solvent.

As to the thiourea, it is possible to substitute simple alkyl derivatives thereof such as methyl or ethyl thiourea and such are intended to be included by the term thiourea as used herein.

In addition to the indicated glycolic acid and thiourea, it may sometimes be desirable to include an appropriate wetting agent as is well understood such as the various well known sulfonic acids an su onates prepared from 1 fatty alcohols, fatty acids, fatty acid esters, and so on. 1 Corrosion inhibitors also may be used, as well understood, when desired to protect other parts such as cadmium- 2 plated steel rivets, bolts and other corrodible parts.

From the standpoint of an acid acceptable for the present purpose, I have found that glycolic acid is the only acid which is completely non-corrosive to aluminum and aluminum alloy surfaces, as above described, which will at the same time remove oxides, tarnish, other soil, staining, light corrosion as distinguished from heavy corrosion, and the like from aluminum and its alloys, without attacking the metal itself, which is non-oxidizing, and which has the required pH of about 2.0 to 2.8 in all concentrations of about 5% to 25% in the water solution.

The product in the various forms described may be packaged dry, the acid and thiourea appearing as crystals, the thickener, such as methyl cellulose, also appearing in dry form if used. Where the product is packaged dry, the required proportions of water will, of course, be added subsequently, such as at the time of use, in accordance with instructions which could appear on the package. Otherwise, the product may be packaged in solution form ready for use.

I claim as my invention:

1. A method for polishing aluminum and aluminum alloy surfaces which comprises the steps of: applying to such a surface a coating of a solution consisting essentially of one part thiourea and one to about five parts of glycolic acid in water providing a pH value between about pH 2 and pH 3; working such solution upon the surface to remove oxides and tarnishing constituents; and rinsing the treated surface with water.

2. A method as in claim 1 wherein the water of the cleaning solution amounts to about ten to twenty parts.

3. A method as in claim 1 wherein said solution remains in contact with said surface from about 15 minutes to 30 minutes to eifect a thorough brightening thereof.

4. A method as in claim 1 wherein said solution contains a dissolved thickener retarding evaporation and runoff of the solution during the contact time.

5. A method as in claim 1 wherein the glycolic acid is present in the proportion of about three parts to one part of thiourea.

6. A liquid composition for polishing aluminum and aluminum alloy surfaces consisting essentially of: 1 part of thiourea; from 1 part to about 5 parts of glycolic acid, and from about 10 to 20 parts of water, said composition having a pH of between about 2 and 2.8.

7. The composition of claim 6 including a minor proportion of a thickening agent dissolved therein, wherein said composition has an increased viscosity and ability to be retained on the surfaces treated.

References Cited in the file of this patent UNITED STATES PATENTS 1,608,622 Schmidt et al. 2.521.122? Nov. 30, 1926 1,736,118 Gravel 1 Nov. 19, 1929 2,049,517 Saukaitis Aug. 4, 1936 2,186,579 Dubpernell et zit/3-1. Jan. 9, 1940 2,270,712 Wood "am-=11: Jan. 20, 1942 2,318,559 Percival May 4, 1943 2,383,800 Johnson "2-226 Aug. 28, 1945 2,511,988 Myers "1522" June 20, 1950 2,558,167 Beghin et a1. "92126" June 26, 1951 2,586,331 Hill "Lu 2 X- Feb. 19, 1952 2,593,259 Brissey et a1. Apr. 15, 1952 2,607,722 Kreml "in!!!" Aug. 19, 1952 (Other references on following page) 5 6 UNITED STATES PATENTS OTHER REFERENCES 2,628,199 Inwenheim ,Z& !i Z Feb. 10, 1953 Corrosion Handbook b Uhlig, page 910, pub. b John 2,672,449 Snell et a1. 'L Q Mar. 16, 1954 Wiley, N.Y. (1948).

913,121 Germany "5.211. 22" June a, 1954 1,038,238 France 1 11 May 6, 1953 161,565 Australia f.'... Mar. 1, 1955 

6. A LIQUID COMPOSITION FOR POLISHING ALUMINUM AND ALUMINUM ALLOY SURFACES CONSISTING ESSENTIALLY OF: 1 PART OF THIOUREA; FROM 1 PART TO ABOUT 5 PARTS OF GLYCOLIC ACID, AND FROM ABOUT 10 TO 20 PARTS OF WATER, SAID COMPOSITION HAVING A PH OF BETWEEN ABOUT 2 AND 2.8. 